In oil and gas production operations, water often is produced concurrently with oil and/or gas. The-rate of water production relative to oil and/or gas is determined by the relative permeability characteristics of the reservoir rock and the relative saturation of water contained therein. In many oil and gas production operations, it is not uncommon to have the percentage of water production, or water cut, in the range between 50% to 90% of the total fluids produced, or more. High percentage of water cut is often observed during the mid- or later-stage of the primary production after water breakthrough. A substantial increase in the water cut of the produced fluids is often observed during the so-called secondary recovery operation processes, in which large amounts of water are injected via a pumping means or a naturally occurring mechanism such as pressure differential or gravity heads from the surface into the subterranean formation to maintain reservoir pressure and sustain oil and gas production.
Most subterranean waters contain large amounts of alkaline earth metal ions, such as barium, strontium, calcium, and magnesium. Water injection during the secondary oil recovery operations also dissolves such ions from the reservoir rocks and brings them to the surface. Under the reservoir conditions, these alkaline earth metal ions can co-exist in a thermodynamically stable state with many anions, such as sulfate, bicarbonate, carbonate, phosphate, and fluoride, etc. However, when the subterranean waters are brought up to the surface during the production of oil and gas, the stable state may no longer be maintained, mainly due to temperature and/or pressure changes. Such a change of solution condition often causes the alkaline earth metal ions to form inherent deposits, or scales, with many of the anions. The presence of barium sulfate often represents a unique and particularly troublesome problem because barium sulfate has a very low solubility. At room temperature, or about 25 degrees Celsius, the solubility of barium sulfate is only 2.3 milligrams per liter.
Another problem associated with the formation of the barium sulfate scales, or any other alkaline earth scales, is that radium, another member of the alkaline earth group of metals, also tends to be deposited at the same time. Disposal of such radioactive solid wastes becomes a serious problem in the oil and gas production operations. Such radioactive waste may be referred to as naturally occurring radioactive material (NORM).
Using the example of a typical oil production field which produces about 100,000 barrels of oil per day at a water cut of 50% at the surface, the amount of barium scale produced can be as high as 60 pounds per day. Continued oil production operation inevitably results in higher water cut and a greater amount of barium sulfate scale. Although only a very small amount of radium is deposited with the barium scale, the entire solid waste mass must be considered radioactive, as far as solid waste disposal is concerned. The expense to be incurred to dispose such radioactive solid waste is enormous, if one is fortunate enough to find a site willing to accept its disposal.
Scale, including NORM, forms in wells and production facilities as a function of the temperature and pressure changes associated with producing hydrocarbons and/or the mixing or commingling of incompatible waters, e.g. waters high in barium with waters relatively high in sulfate. As shown in FIG. 3, barium sulfate becomes more soluble when heated. In addition, it also becomes more soluble when the ionic strength (salt content) of the solution is increased. Both of these conditions can be obtained by using the produced waters from a hot salt water producing well. Typically, these wells would be producing wells in the oil and gas field that produce significant amounts of associated water. The heat energy of the well is used to increase the solubility of the barium sulfate in the presence of salt water. Steam is not a viable alternative since solids are not practically soluble in steam.
Various proposals have been made in the prior art for the removal of barium sulfate scales using chemical scale removal compositions. Examples of barium sulfate scale removal techniques can be found in U.S. Patent No. 2,877,848; U.S. Pat. No. 3,660,287; U.S. Pat. No. 4,708,805; U.S. Pat. No. 4,190,462; U.S. Pat. No. 4,215,000; U.S. Pat. No. 4,288,333; U.S. Pat. No. 4,973,201; and U.S. Pat. No. 4,980,077. All these prior art technologies are designed to remove scales from equipment or tubular goods, such as meters, valves, tubing strings, surface pipes, etc. None of the prior art addresses the issue of the disposal of the radioactive solid waste, nor is any prior art known. Also, the use of chemical scale removing agents is subject to a large number of variables. They usually require a right combination of environmental variables in order to work, and yet even under the right conditions they do not always work. Furthermore, since a large amount of solution is required to dissolve the scale, it is essentially economically prohibitive to use such chemical means in the waste disposal process. The techniques proposed in the prior art are to be used for spot-wise dissolution of scales formed on pipes or other equipment, but they are not suitable for handing solid waste disposal.
U.S. Patent No. 4,973,201 discloses a method for decontaminating surface layers of the earth which are contaminated with precipitates of alkaline earth metal sulfates including radium sulfate. The method includes applying an aqueous chemical composition comprising a chelating agent and a synergist to the surface layers in situ to bring the precipitates into dissolved form after which the dissolved precipitates are leached into lower layers of the earth by percolation with water.
U.S. Pat. Nos. 4,980,077 and 5,049,297, which are assigned to the same entity as the '201 patent described above, generally disclose a method and composition, both utilizing a chelating agent, for removing barium sulfate scale deposits from oil field articles.
U.S. Pat. No. 5,022,787 discloses a method for disposing of noncondensing and toxic geothermal gases wherein the gases are returned to the underground.
U.S. Pat. No. 4,632,601 discloses a system for disposing of non condensable gases from geothermal wells wherein the non-condensable gases are dissolved into geothermal waste water.
U.S. Pat. No. 4,429,740 discloses a gas producing well wherein waste water is disposed of in an earth formation underlying the gas-producing earth formation.
U.S. Pat. No. 4,400,314 discloses a method for disposing of high level radioactive water wherein an aqueous solution is diluted with formation water recovered from a subsea reservoir in a porous geological formation and the dilute solution is injected into the geological formation.
U.S. Pat. No. 4,738,564 discloses a method for disposal of nuclear and toxic wastes wherein the wastes are rendered harmless by dilution into a huge mass of molten lava.
Finally, U.S. Pat. No. 4,844,162 discloses a method of treating a flow of hot, pressurized, hydrogen sulfide-containing geothermal steam. This method teaches disposal of condensate in a disposal well but offers no suggestion or even consideration of the difficulties involved in the disposal of solid NORM.